The last piece of work to be completed by the decay-tube contractor has been the vacuum test. After the completion of the tube, the installation of the exit window and the (provisional) entrance window, a roughing pump and a roots pump were installed in the service gallery and connected to the decay tube.

Pumping down of the 4500 m³ volume took a bit more than 30 hours. The goal was to reach a pressure of 1 mbar, but the ROOTS pump used below 30 mbar proved to be very efficient - finally, the pumping down was stopped below 1.5x10^-2 mbar.

At this point, the valves were closed and the pumps stopped. A minor air inlet to reach 1 mbar was performed. The leak test consisted in observing the pressure rise (which could be due to a leak, but also be due to outgassing) during a period of 10 days. During this period, the pressure varied (instrument accuracy, temperature change?) but never exceeded 1.4 mbar. Conclusion: The decay tube is leak-tight.

  Pumping the decay tube

  Leak test (pressure increase) during 10 days